Cable routing. People lose their minds over were their cables should be attached to the frames and how to actually attach them. It's yet another minutiae of the sport that makes it what cross so intriguing. From obsessing over tire pressures and casing count or the best way to run a single ring to sourcing obscure brake pads, it's a typical type of obsession that is so prevalent in the sport. I'm as bad as anyone else. I'll spend who knows how many hours and dollars "fixing" something so very tiny and then waste myself and my equipment in an hour long slog fest through the worst gunk and grime possible. If that doesn't kill the bike, I throw it on the roof rack for a four and a half hour drive home in the driving NW rain. So you can see how it makes perfect sense to invest in unobtanium bits and maintenance heavy components(this is a sarcastic comment, in case your internal meter is off). Over the last few years I've come to add weight back to the bike to get reliability up, and perhaps more importantly, time spent wrenching down.
This leads me to cable routing. I've ran all the configurations there are and can honestly say they all work pretty well and all have things that suck enough to drive one crazy. I'll also come out and admit that I don't think there is any "magic bullet" when it comes to cable routing. You'll wind up running whatever works best for you and has the least annoying downsides. I'm just going to lay out what I think are the pros and cons of each. I'm only going to go over how they run in regards to the frame routing. I'll leave the front brake routing's for another day as that is a topic unto itself. So here we go.
Down Tube Routing
1. It's the lightest as there is the bare amount of housing and only one cable stop per cable.
2. You don't need a pulley to run a road front derailleur. Road front derailleurs work better than mountain front derailleurs. It has to do with the leverage of the arm in relation to the cable pull of the shifters, plus the shape and profile of the cage. Bottom line, you can make a mountain front derailleur work OK, but not as well as a road front derailleur. Since no one makes a top pull road front derailleur, the only way to get one to work is either pave a pulley under the front derailleur on the seat tube which loops the cable back up to the derailleur, or run down tube routed cables. Never ind if you run a single ring. (Yet again, another posting and topic.)
3. No loop of housing at the rear derailleur pointing up at the sky for grit and grime to accumulate in a make shifting suck. Because the housing actually points down, it never fills up with slop and gets sticky. Or at least it takes a lot longer for it to happen.
4. It looks all classic and traditional. Not very MTBish.
1. The cables are a pain when it comes time to grab your down tube. They kind of shift around and feel like they are going to pinch your fingers as you're shouldering your rig.
2. More grass and crap seems to hang from them when it gets really messy. No big deal really, but it can make cleaning the bike take longer.
3. It looks all classic and traditional. Not very MTBish.
Top Tube Routing
1. It keeps the cables out of the way. Of your hands, of the slop coming off your front tire, everything.
2. It makes cleaning the bike a bit faster. This matters if you've a limited time to spend cleaning and wrenching before your next race. It really matters when you are in the pits and wrenching for someone else. I wonder if that's why you see so many PRO rigs running top tube routing.
3. It makes for a cleaner and smoother shoulder. I don't mean cleaner in that your gloves won't get dirty, but that the motion won't have any hanging up on cables.
1. The afore mentioned front derailleur woes. No big deal if you run a single front ring, but a bit of a pain if you run a double. If you run a triple, stop reading this and go out and remove your helmet mirror, seat bag, and water bottle cages. What you do isn't racing. This is about race bikes. An aside on the pulley. I don't mind them so much, but they are another moving part that has to be dealt with and on certain frames, can limit tire clearance at the seat tube.
2. Unless you like changing your cables every week (Yeah, that's right. Every week.) or run a sealed system like Gore, your shifting will suck balls in a very short amount of time. It happens because of that last loop of housing at the rear derailleur points up at the sky allowing all kinds of crap to run down the cable and deposit itself in the housing. Even though the cables are up and out of the way, any grime and slime that gets on the bike will be funneled right down into that last bit of housing. That's why pros get new cables and housing every week.
I've come to run the rear brake and rear derailleur on the top tube and use sealed cables. I like running a road front derailleur so my front derailleur cable goes on the down tube. I had the guy who built my bike use a single split stop at the 6:00 position on the down tube which keeps it as tucked up to the frame as possible. I think this offers me the right balance of performance and limited maintenance. If I had more limited wrenching skills (they're way mad) and never wanted to change cable and housing, then I'd have the cable routed on the down tube. If I had huge ducketts, I'd run regular cables on the top tube with a pulley and have my personal mechanic deal with changing them out on a constant basis. As is, I think running a sealed system is the way to go. It costs more up front, but the shifting lasts the whole season. Just don't use that Nokon crap. I'd have to hit you.